Differences Among Virus Populations Recovered from Mice Vaccinated with an Attenuated Strain of Venezuelan Equine Encephalomyelitis Virus

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Abstract
Summary: The differences among virus populations that were recovered after the vaccination of mice with an attenuated strain of Venezuelan equine encephalomyelitis virus were investigated. Large quantities of virus were recovered from spleen and liver tissues; smaller amounts were isolated from the brain. During the first 24 hr, virus populations recovered from all three tissues contained a majority of nonlethal, immunizing particles which gave higher titers in L cells than in mice. Such virus preparations resembled the attenuated vaccine virus strain with respect to the degree of virulence for mice and in the small size of the plaques produced on chick fibroblasts. At 72 hr, virus particles lethal for mice by the intraperitoneal (i.p.) route appeared in spleen tissue and a small percentage of plaques, larger in size than those produced by the attenuated strain were found. Such virus populations, therefore, showed properties similar to those shown by the virulent parent virus from which the attenuated strain was derived. In contrast, liver and brain tissue taken from vaccinated mice yielded only virus which was nonlethal when tested i.p. and which produced only small plaques. Despite the presence of virulent virus in spleen, demonstrable after subinoculation, no signs of clinical illness were observed in any of the vaccinated mice.